Automobiles and trucks receive exterior finishes for several well known reasons. First, such finishes provide barrier protection against corrosion.
Second, consumers prefer an exterior finish having an attractive aesthetic finish, including high gloss and excellent DOI (distinctness of image).
A typical automobile steel panel or substrate has several layers of finishes or coatings. The substrate is typically first coated with an inorganic rust-proofing zinc or iron phosphate layer over which is provided a primer which can be an electrocoated primer or a repair primer. Optionally, a primer surfacer can be applied to provide for better appearance and/or improved adhesion. A pigmented basecoat or colorcoat is next applied over the primer. A typical basecoat or colorcoat comprises a pigment, which may include metallic flakes in the case of a metallic finish. In order to protect and preserve the aesthetic qualities of the finish on the vehicle, it is well known to provide a clear (unpigmented) topcoat over the colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering.
Coating compositions comprise one or more film-forming polymers. Most commonly, linear polymers are employed that cure, upon application, by reaction with crosslinking agents. However, the use of non-linear graft copolymers has been disclosed. For example, U.S. Pat. No. 4,801,653 to Das et al. describes the use of hydroxy functional graft copolymers. Das et al. disclose grafting by a condensation reaction between epoxy groups of a glycidyl ester, contained in an acrylic polymer, and carboxy groups on at least a portion of vinyl monomers which are polymerized in the presence of the acrylic polymer.
In preparing graft polymers in general, various living polymerization methods have been disclosed for obtaining functional ended polymers by selective termination of living ends. Such functionally ended polymers may subsequently be attached to another polymer, that is, as so-called macromonomer "arms" on a polymeric backbone to form a comb graft copolymer. Webster, in "Living Polymerization Methods," 251 SCIENCE 887 Feb. 22, 1991) generally discloses living polymerization methods for preparing architectural forms of polymers, including graft and comb copolymers.
Graft copolymers containing carboxyl groups and the preparation of these polymers is shown in Japanese Laid Open Patent Application (Kokai) No. 1-182304 dated Jul. 20, 1989. This reference shows graft copolymers that have carboxyl groups based on acrylic and methacrylic acid in their side chains that have hydrophilic properties and teaches the use tertiary alcohol-based ester units of acrylic or methacrylic acid to form a macromonomer which is used to form a graft copolymer and then is hydrolyzed to form carboxylic acid groups on the polymer. The process taught by the reference is an inefficient process which does not form pure graft copolymer but results in a mixture of graft copolymer and low molecular weight components that are detrimental to pigment dispersions formed from the graft copolymer and finishes formed from such a composition.
U.S. Pat. No. 4,680,352 to Janowicz et al. and U.S. Pat. No. 4,722,984 to Janowicz disclose the use of cobalt (Co) chelates as chain transfer agents in free radical polymerization. The latter patent discloses that macromonomers prepared by cobalt chain transfer can be polymerized to produce graft copolymers which are useful in coating and molding resins, including high solid finishes and aqueous or solvent based finishes, although the use of such polymers have so far found only limited use in the finishes area. The use of chain transfer agents to prepare dispersed polymers used in automotive coatings is disclosed in U.S. Pat. No. 5,010,140.
The present invention relates to aqueous coating compositions.
The evolution of environmental regulations has led to the need for products with lower volatile organic content (VOC). Water dispersible polymers are well known in the art and have been used to form water-based coating compositions, pigment dispersions, adhesives and the like. Much of the prior art, however is directed to either latex or water reducible linear polymers. Latex-based systems tend to have problems with coalescence. Linear water reducible polymers are difficult to process because of high molecular weight.
BASF EP 0363723 describes an acrylic copolymer dispersion for use in an OEM clear coat to be crosslinked with a melamine formaldehyde resin. The linear acrylic copolymer is prepared in a solvent in a two-stage process where the hydrophilic part (acid-functional monomer) is concentrated in one of the two stages. The overall copolymer is afterwards neutralized with an amine and dispersed in water. The difference from a one-stage product is that the solids/viscosity relation is more favorable. A disadvantage of this technology is the fact that the hydrophilic part needs to be over 60% of acid-functional monomer which could give problems in humidity resistance.
Bayer patents EP 0218906 and EP 0324334 describe the synthesis of hydroxy-acid functional acrylic copolymers prepared in solution before neutralizing with an amine and dispersing in water. Bayer EP 0334032 describes the synthesis of an acid functional urethane oligomer which is used to stabilize a WB acrylic copolymer dispersion. AKZO U.S. Pat. No. 5,098,947 describes urethane modified acrylic copolymer dispersions for waterborne coatings.
Regarding the presently claimed waterborne composition, branched copolymers allow for higher molecular weight polymers at lower viscosity compared to linear polymers of the same molecular weight. Such branched copolymers also allow for segmentation of the copolymer into hydrophobic and hydrophilic portions for improved coating properties. Accordingly, the branched copolymers of the present invention are hydrosols that form a dispersed phase at high pH and high water content while forming a solution polymer at low pH and/or high organic solvent/low water content. This ease of inversion can provide coatings with better pin-hole resistance among other desirable properties. Applicants have found that coatings made from such aqueous branched copolymers are hard, water and humidity resistant, and show excellent humidity characteristics.